There is a global interest in the development of alternative energy sources especially of wind power. The idea of harnessing the “free” energy in the air by using the passing wind to rotate a shaft in order to produce useful work has long been studied. During the past several years, however, the recognition of the limited supply of fossil fuels and the soaring costs of energy in general have created a renaissance in wind turbines all seeking to extract energy from the passing air with sufficient efficiency to constitute practical sources of electrical and mechanical power.
In its simplest form, a wind turbine comprises a shaft which carries blades or other means of catching the wind and rotating the shaft from which mechanical or electrical power is generated. Within given limits the velocity with which the shaft rotates is roughly proportional to the velocity of the wind acting on the shaft's rotators and to the amount of energy produced. The faster the shaft rotates for a given velocity of wind and a given load, the greater is the efficiency with which wind energy is converted into mechanical or electrical energy.
It has thus been one approach to increasing the efficiency of wind generating machines to increase the efficiency of the rotation of the working shaft for a given wind velocity.
The electrical power generated by wind turbines is often transmitted considerable distances to centres of population, and one reason for this is that people usually live in sheltered terrain where the wind resource is more modest. The less concentrated energy in the wind in such areas reduces the economics behind its energy capture with conventional wind turbines.
Several technical papers address these issues and cover the period between 1978 and 2006. Several patent documents use the word Venturi to describe the mechanism employed in the patent documents to improve the power output of the turbines. These patent documents cover principally hydraulic applications and no performance data is provided with the exception of a Japanese patent application for a wind turbine which employs a diffuser and brimmed inlet.
The application of diffusers to augment the performance of wind turbines has been on-going since the early 1970's. This technique lead to the acronym DAWT or Diffuser Assisted Wind Turbine. In most studies, the largest wind augmentation has been an increase in wind speed of 240%. This was obtained with a diffuser having a length to diameter ratio (L/D) of 4.5 to 1. Most modern turbines have very large diameters of the order of 90 meters. An increase in the wind speed of 240% would allow the use of a smaller rotor of say 45 meters. However the length of the required diffuser would be an unpractical 202.5 meters (45 meters×4.5=202.5).
As the three-bladed Horizontal Axis Wind Turbine (HAWT) is easily the world's accepted wind generating technology, it is evident that this type of turbine would be used in any DAWT application or experimentation. In fact, most of the documented experimental work to date with the intent of producing electrical power has been performed using a HAWT. Two papers refer to the use of a Savonius rotor. In spite of the acceptance of the HAWT, the Applicant could not find any reference to any experimentation using a convergent, ducted turbine tunnel and divergent for any HAWT application. Evidently, it appears that little or no work has been done to develop a new generation of wind turbines that would perform better than an HAWT or Savonius turbine in an augmented fluid stream.
In spite of the huge world demand for renewable wind energy there are no commercial applications of either diffuser-augmented, nor convergent-augmented, nor convergent-divergent augmented turbines. The reason is quite simple. To date, it has been much more cost effective to increase the swept area of the rotor to increase the power output than to increase the wind energy density using augmentation devices and use a smaller diameter rotor. As a corollary to this statement, one could add that no one has succeeded in increasing the wind energy density high enough to justify using augmentation devices and a smaller turbine rotor.
A first known prior art document is available on the Griffith University web Site and is a 1998 thesis entitled “Evaluation of self starting vertical axis wind turbines for standalone applications”. In spite of an exhaustive literature review, no reference is made to using a convergent/divergent to augment VAWT performance nor is there any mention of using a diffuser or turbine ducting to increase performance. The vertical axis wind turbine is possibly the second most common commercial wind turbine. It would appear that everyone figures that if augmentation technology will not work with a HAWT, it will not work with any other type of turbine. Since publishing this thesis, the author and others have been working on a DAWT for hydraulic applications but again they are using a diffuser and a brimmed inlet without a convergent section.
A second known prior art document is a “Wind Engineering” dissertation paper from 2004 by the Ashikaga Institute of Technology entitled “Wind Tunnel Analysis of Concentrators for Augmented Wind Turbines”. The abstract, line 3, discloses: “However most of the studies of the ducted rotor concern the effect of the diffuser while little research has been done concerning the concentrator (nozzle) This paper analyses the effect of the concentrator and its optimum design.” It can be observed in this work that there is no talk of a diffuser and the wind turbine is a standard HAWT.
A third known prior art document is a “Journal of Energy” 1978 publication entitled “Fluid Dynamics of Diffuser-Augmented Wind Turbines”. This paper by Gilbert B L, Oman R A and Foreman K M represents pioneering work performed on the development of a DAWT. Anyone who writes on this topic typically includes this paper as a reference. In the abstract, it is disclosed that “This first generation of DAWT can provide about twice the power of a conventional WECS with the same turbine diameter and wind.” At this early stage, there was no use of a concentrator or ducting. A conventional HAWT was used. This work was completed when the diameter of a HAWT was much smaller than today as the composite materials used today were not available. In the 1980's, a company was created to use this technology. It went bankrupt while more or less convincing everyone that the future was in large diameter non-augmented turbines.
A fourth known prior art document is a publication from the “13th Australian Fluid Mechanics Conference” in 1998 entitled “Computational Modelling of Diffuser Designs for a Diffuser Augmented Wind Turbine”. The work presented in the paper is performed using a brimmed inlet, diffuser and HAWT.
A fifth known prior art document is a publication of the “International Journal of Energy”, in 2005 by CES, Indian Institute of Technology, entitled “Air concentrating nozzles: A promising option for wind turbines”. In the conclusions of this paper, on page 411, second last paragraph, it is disclosed that “The greatest percentage improvement in the static torque by the use of the convergent nozzles occurs at low wind speeds.” This is not an acceptable result as the large size of the equipment required to generate an equivalent amount of power obtainable at a higher wind speed will be uneconomical. The researchers did not use a divergent or ducted turbine tunnel and the trials were performed at very low wind speeds.
A sixth known prior art document is a publication from the “Second International Symposium on Wind Energy Systems”, in 1978 by Pahvali University, Iran, entitled “Power Augmentation in a ducted Savonius rotor”. In the summary, paragraph 1, it is disclosed that “Several ductings, concentrators and diffusers are examined and their effects on the performance characteristics of a split S Savonius rotor are presented in this paper.” In the introduction, at point i, it is also disclosed that “While concentrators of the type (a) in FIG. 2 are quite ineffective, those of the type (b) in the same figure yield considerable improvement in the rotor performance.” The conclusion of this report is that one convergent and two divergent sections are the most efficient combination for the Savonious turbine used. It also concludes that the concentrator should be offset with respect to the centerline of the ducted tunnel. However, current experimental work shows that at no time would it appear even remotely practical to use one convergent and two divergent sections. One can note that the improvement in performance is more related to the impingement of the air stream than the performance of the convergent and divergent sections themselves.
A seventh known prior art document is a publication of the “Trans Japan Society for Aeronautical Space Science”, in 2006 by Kyushu University, Kasuga Japan entitled “Development of a High Performance Wind Turbine Equipped with a Brimmed Diffuser Shroud”. The paper discloses the following (second phrase paragraph #1 and the last line of page 19):                3.1 Selection of a diffuser-type structure as the basic form        “We examined the flow of three typical hollow structures as shown in FIG. 2 namely a nozzle-type model that reduces the inside cross section, a cylindrical-type model that has a constant inside cross section and a diffuser type model that expands the inside cross section downstream.        As seen in FIG. 4(a) the wind tends to avoid the nozzle type model while the wind flows into the diffuser type model as it is inhaled as seen in FIG. 4(b).”        
Essentially, this work suggests that nozzles and convergent sections should not be used as augmentation devices. This is the opposite to the findings of some current experimental work carried out by the Applicant.                “3.2 Improvement of acceleration performance of the diffuser structure by adding peripheral appendages (the first line of the second paragraph and the last paragraph of page 20):        “As a result of several attempts it was found that the wind speed is increased by adding an appropriate entrance (called an inlet shroud) and a ring type flange at the exit periphery (called a brim, see FIGS. 8, 9, 11 and 12) to the diffuser body. The effect of the inlet shroud is found in the following point. It restrains flow separation at the entrance fairly well and the wind flows in more smoothly.”        
This implies that the inlet shroud serves to minimise head loss at the entrance by reducing flow separation around the mouth of the inlet, as it does not increase the static pressure and as such is not an augmentation device. A nozzle and a convergent section are augmentation devices as they are designed to increase static pressure.
Other prior art patent documents known to the Applicant include U.S. Pat. No. 7,094,018, US 2007/0020097 A1, EP 0 935 068 A2, GB 2 430 982 A and U.S. Pat. No. 6,756,696 B2.